When we think about military service, we tend to think of a handful of wars/"conflicts": Wars in the Middle East [from the Gulf War to present], Viet Nam, Korea, WWII, WWI and the Civil War.
Some tend to forget the war that gave us our independence - The American Revolution.
Others that frequently get overlooked: War of 1812, The Indian Wars [c1865-1890s], The Seminole Wars, Mexican War, Spanish-American & Philippine-American Wars, French and Indian War & Colonial Wars.
Technically, any war predating the American Revolution would not be a US-involved conflict, but they still involved our ancestors!
When you start researching your military ancestors, start early - the mid-1600s and go from there.